Bicarbonate and potassium regulation of the shape of Streptococcus mutans NCTC 10449S

Abstract

Morphological changes of S. mutans NCTC 10449S associated with growth in modified Jordan medium and FMC medium (Terleckyj et al., Infect. Immun. 11:649-655, 1975) were studied by scanning electron microscopy. The cells were bacillary in Jordan medium, but coccoid and of unequal size in FMC. Transfer of the cells from Jordan medium to FMC and vice versa reversed their shapes, as did salt exchange between these media. Morphological changes could not be ascribed to either medium pH, concentration of P, or Na+/K+ ratio. However, they were growth dependent, since the changes did not occur when the cells were suspended in salt components alone or in media supplemented with protein synthesis inhibitors. Only a high bicarbonate/K+ ratio, as in FMC, produced spherical cells, whereas cells remained bacillary in medium with a low bicarbonate/K+ ratio, as in Jordan medium. Manipulating this ratio in other media resulted in similar shape changes. Thus, the shape of S. mutans 10449S can be dictated by the ratio of bicarbonate to K+ in the growth medium.

Factors influencing cell shape in the mutans group of streptococci

“…These values are different from one another (P < 0.001). As previously reported (14), the widths of rods were less than those of either cocci or ellipses.…”

“…When the ratio is low (0.008), as in the mnodified Jordan complex medium (JM; 9, 13), the cells are bacillary; when it is high (2.84), as in the defined FMC medium (15), the cells are coccoid (14).…”

“…Our previous study (14) demonstrated that the shape of Streptococcus mutans NCTC 10449S can be changed from bacillary to coccoid, and vice versa, by adjusting the salt contents of a variety of complex and defined culture media in which it is grown. Within wide ranges, the shape of this cell is not dependent on the absolute concentrations of HC03-K+, Na+, or phosphate; on the ratio Na+/K+; or on the addition (or deletion) of iron, magnesium, or manganese salts to (or from) media.…”

“…Growth rates of 10449S were examined in six culture media: both JM and FMC, reformulations of JM and FMC made by exchanging their salts (14), and reformulations of them made by substitution of B4072-for nearly all of the HC03-in FMC and by addition of B4072-to a high level in JM containing low K+. By these procedures, JM and FMC were formulated to have the HCO3-/K' and B4072-/K+ ratios detailed in Table 3.…”

“…Cells were examined by Gram stain and scanning electron microscopy, as previously described (14). Cell shapes are reported in terms of average length/width ratios (-+standard deviation) in Table 1.…”